Laser active optronic systems have numerous applications, including in particular telemetry, based on the flight time measurement of a laser pulse emitted by the system and backreflected by the target, or active imaging systems in which a target to be imaged is illuminated by a non-natural source of the laser source type. For eye-safe reasons, it is necessary to avoid the use of emission sources whose wavelengths are in the visible. Eye-safe wavelength emission sources are preferred, that is to say those having wavelengths at which the regions of the eye anterior to the retina (the cornea, the aqueous humor and the lens) are absorbent so that the retina is protected from the impact of a laser beam in the eye. These wavelengths belong to the near infrared (typically above 1 μm) and the sources conventionally used are for example erbium-doped lasers (1.5 μm emission wavelength) or neodinium-doped lasers (1.06 μm emission source) which are associated with nonlinear optical devices such as optical parametric oscillators, in order to emit at wavelengths above 1 μm. The use of such sources requires, for the optronic systems, components (optic, receiver, etc.) that are sensitive to these wavelengths.
One reason for the insufficient detectivity in active optronic systems, of the active imaging or telemetry type, stems especially from the parasitic flux incident on the detector that is generated by atmospheric scattering over the first one hundred meters or so of the optical path between the system and the target. This parasitic flux may generate a detection signal of amplitude greater than that resulting from the flux backreflected by the target, which may be several kilometers from the system.
One way of obviating this problem consists in switching the detection capability of the receiver of the optronic system in order to make it inoperable over a given duration, by installing an electronic device in the detector itself. This technique does not depend on the wavelength used—it is therefore operative in eye-safe optronic systems. However, it may be necessary to obtain very short switching times, for example for the construction of imaging systems with distance resolution. In this case, the electronic switching device must have a large bandwidth and is a noise generator.